1. Field of the Invention
The present invention relates generally to honeycomb labyrinth seals and, more specifically, to an improved seal which is used for low-pressure steam turbines.
2. Description of the Related Art
A grooved honeycomb labyrinth seal for steamed turbines is described in U.S. Pat. No. 4,416,457 (assigned to Westinghouse Electric Corporation).
Referring to FIGS. 1 and 2 herein, the honeycomb labyrinth seal 3 described in the aforementioned U.S. patent is formed from a plurality of arcuate segments each comprising a base portion 7 and a plurality of rows of honeycomb cells 9 extending radially inwardly from the base portion 7 so that each cell 9 is open adjacent the blades 1. A plurality of passages or grooves 11 are so disposed that each cell 9 is connected to at least one passage, while the majority of the cells are connected to two passages or grooves 11. The groove 11 on the left side of the drawing is open to the upstream steam while the groove 11 on the right side of the drawing is open to the downstream and there is an interconnection between the grooves 11 and the cells 9 allowing steam and water entrained in the steam to flow across the base 7 of the seal from the upstream to the downstream side of the seal. The cells 9 are generally hexagonal having six walls, a plurality of which are common with adjacent cell walls.
An axially disposed gap 13 is disposed between the circumferentially adjacent honeycomb labyrinth seal segments which permits water collected in the grooves 11 to drain therefrom. An appropriate drain, not shown, is also disposed in the blade ring 5 to drain the water from the seal locating groove 4 in the blade ring 5.
FIG. 3A shows a typical arrangement for existing honeycomb seal designs in a steam turbine blade path. A radial step is formed between the seal inside diameter and the outer diameter of the downstream blade 16. A radial channel is formed between surface 12 and surface 12A for the purpose of removing steam and moisture from the blade path flow. It is difficult to achieve efficient turning of the flow leaving the rotating blade tip region (under the seal) and thus it is difficult for the moisture to enter the extraction slot. This can lead to disturbed flow conditions in this region.
It is postulated that moisture drainage from the seal may follow a generally axial path and become entrained into the main downstream flow through stationary blade 16, leading to increased moisture erosion damage to the downstream rotating blades.
By observing flow field behavior and moisture erosion characteristics downstream of a typical honeycomb seal installation, it has been determined that a disturbed flow pattern (standing vortex) can exist in association with the radial step between the inner diameter of the seal and the outer diameter surface at the inlet to the downstream stationary blade row. Also, moisture collected by the seal may not be flowing into the downstream moisture removal or extraction slot as intended. Both of these phenomena may be attributed to a limited ability of the extraction steam and water flow to negotiate the sharp 90.degree. corner at the seal exit edge.